Impression material and method of -



United States Patent 2,733,157 Patented Jan. 31, 1956 IMPRESSIONMATERIAL AND lVIETHOD OF MAKING SAME John A. Cornell, Upper MerionTownship, Montgomery County, and Harry E. Kemrnler, Elkins Park, Pa.,assignors to H. D. .lusti & Son, Inc., Phiia'iieiphia, Pih, acorporation of Pennsylvania N Drawing. Application September 22, 1953,Serial No. 381,736

12 Claims. (Cl. 1ll638.5)

This invention relates to impression materials for taking impressions,pertaining particularly to materials for taking impressions of parts ofhuman beings, such as of the mouth for dental purposes and of the outerear for aural purposes, and constitutes a continuation-in-part ofapplication Serial No. 283,984, filed April 23, 1952.

In taking dental impressions, for instance, according towell-established practices, the impression material mass is mixed to itsattained consistency, it is applied to a support, such as a tray, isinserted in the patients mouth in forced intimate contact with the teethand gums to be reproduced, is held in such contact until set-up of themixture in the imparted form is completed, the tray-mounted mass is thenwithdrawn to form an integral mold from which requiredly accurateplaster-of- Paris or artificial stone models can be made.

The fact that the impression material is to be used for human dental andlike impression purposes imposes certain critical and minimumrequirements for an impression material to meet all of the factorsencountered with patients in dentistry. These requirements of the idealimpression material are as follows: it must be bland and neither toxicnor caustic in its reaction with human tissue; it must be usable andsettable at substantially mouth temperatures to minimize thermal shocksbefore, during and after setting; it must set rapidly in the mouth to agel of sutlicient dimensional stability adequate to prevent tearing andwith high elasticity and of fine texture, from which very accuratereproductions of the contacted structure of the mouth can be made; itsdimensional stability must be such as to minimize early shrinkage andcontraction; it must have a closely controlled rate of gelation so thatthe mixture will not set after mixing and while preparing the tray, andyet will set in a predictably short period of time, as a result of whichthere is a minimal discomfort to the patient, and with the time rate ofgelation substantially constant for each successive batch or mix, sothat the dentist will quickly recognize and allow for the time factorsnecessary in successful use of the material. A satisfactory impressionmaterial is inert and non-reactive with dental stone or plaster; and itmust have immunity to deterioration during storage, regardless ofincident temperature and humidity conditions, so that it hassatisfactory shelf life. The foregoing requirements are found from priorexperience with known impression materials to involve operating factorswhich may be mutually antagonistic, so that an improvement in one factorwill work detriment to another factor. Some of the desiredcharacteristics such as the setting rate and factors affecting comfort,cannot be controlled within thewide range required for different typesof patients and different kinds of dental manipulations. The order ofimportance of the requirements may vary with dilferent situations andthe foregoing listing should not be considered as one in order ofimportance.

It is an object of the invention to provide an impres sion materialwhich more nearly approximates an ideal impression material and whichwill better satisfy the requirements of this ideal material as indicatedabove, said impression material being available at reasonable cost.

Many efforts have been made to improve impression materials fordentistry. However, each of these eflforts has failed to meet one ormore of the recited essential attributes of the dental impressionmaterial stated above. These efforts, so far as known, are characterizedby certain disadvantageous features militating against the completelysatisfactory use thereof, despite the fact that, perforce, in theabsence of better material, a certain degree of commercial utilizationhas been effected.

In general, so far as actually produced and sold dental impressionmaterials are concerned, in the main they have been of two types orgroups. One group (a) is a reversible type of impression material,typified, for instance, by an agar hydrocolloid compound, and the othergroup (b) is an irreversible type of impression material, typified, forinstance, by an alginic acid compound. These are unsatisfactory withinthe concept for the requirements of the ideal impression material asrecited above for'several reasons.

For instance, the (a) type ofcompounds require extension manipulation,must be heated to be softened to a degree making them usable but withoutraising the temperature too high to complicate the manipulativeoperations before using the material, and thus require accuratetemperature control. The heated wax has a shocking and adverse reactionto the human tissues. Further, in order to make the compounds set, ithas been customary, in the interest of speed, to spray the compound withchilled Water, requiring bulky cooling equipment, and again withdefinitely adverse reactions from the patient. Finally, even when set,the material has small elasticity and inadequate form-retention so thatundercutsand the like tear the impression and militate against itsfidelity at points Where it is or may be most important.

The type (b) alginate compounds are fairly satisfactory as regardsequipment and handling, as well as from the standpoint of the patientand his comfort, but are inherently characterized by otherunsatisfactory characteristics. As is well known, the type (b) compoundsinvolve the gelling of a soluble potassium or sodium alginate in aqueoussolution by the gradual dissolution of a slightly soluble calcium salt,producing calcium ions, 1

which form a gel with the alginate. One of the primary disadvantages ofthis type of material is that, although the formed gel is elastic, inthe sense that ittends to return by its own resilience to its acquiredor imparted form after deformation from that form, as in the removal offormed gel from contact with the teeth or edentulous ridges of apatient, it is not possessed of very accurate dimensional stability, andthe strength thereof is frequently insufiicient or inadequate to preventthe breaking off of thin undercuts or ridges. This type of material isalso one that has an unfavorable reaction with dental stone or plasterwith which it comes into contact, with consequent lack of fidelity ofthe molded form to the impression. Additionally, this material has ashort shelf life, especially in hot and humid climates, and itsdimensional stability suffers by reason of an early contraction of thegel.

The structure of a carboxy-methylcellulose unit has a considerablesimilarity to that of an alginate, although there are some diiferencestherefrom. The main difference is in the repeat distance and thedimethylne ether linkage in the cellulosic material. The composition ofcarboxy-methylcellulose can be closely controlled so that thevariability of composition and properties thereof is less than that ofthe alginate, which is produced from seaweed. A compositionincorporating carboxy-methylcellulose is more uniform than one utilizingan alginate. One of the advantages of carboxy-methylcellulose lies inthe fact that the alkali metal and ammonium salts ofcarboxy-methylcellulose are water soluble. However, ammonium saltsmeasurabily affect the setting of artificial stone or plaster-of-Paris.The alkaline earth salts have limited solubility, are somewhatthixotropic in character and may behave with the unsubstituted hydroxylgroups to form additional compounds which in turn affect the setting ofartificial stone or plaster.

The degree of substitution is important, since it has been found that alower carboxyl substitution in the recurring anhydro glucose units tendsto proportionately inhibit the setting of artificial stone. It hasfurther been found that the highest substitution (1.3 per glucose unit)causes but little interference in respect of the setting of suchartificial stone. This is a factor which prior researchers haveapparently ignored with consequent deficiencies in the performance ofimpression materials incorporating carboxy-methylcellulose.

As a result of extensive tests, it has been discovered that the saltsmost likely to precipitate and form a strong gel withcarboxy-methylcellulose alginic acid are those of the amphotericmultivalent metal elements, such as alummum, chromium, iron, lead,cadmium, zirconium, cerium, and the like. However, there are few saltsof these metalswhich can be practically used because the precipitat1ngion is not readily available and, too, their use must assure anacceptable end product. Lead salts are not acceptable as gel-formingagents for the specific purposes of this invention, even thougheffecting the formation of an excellent gel. This is because (1) oftheir toxicity; and (2) the solubility product of leadcarboxymethylcellulose, versus calcium sulfate supplied by theartificial stone or plaster upon casting, allows the free calcium tomigrate to the gel, and to react undesirably as previously noted. Theuse of basic aluminum acetate is also limited, since the solubility ofthis salt is dependent upon the acetate content in both neutral and acidsolution, and since long storage causes it to lose acetate in the formof acetic acid, thus becoming more difiicultly soluble; its use isunwarranted, and the availability of the precipitating aluminum 1011throughout a homogeneous admixture of the carboxylated gel formingmaterial is therefore limited, because of an essential lack of controlof the amount of precipitatmg ion due to the inherent instability of thealuminum salt mentioned above.

It is an object of this invention to improve impression materials; toprovide impression materials of high elasticity with dimensionalstability; to provide impression materials which can be closelycontrolled as to their time of setting from the initial mixing to thefinal set; to provide at low cost a highly efiicient impression materialfor dental and aural purposes which is possessed of a minimal adverserelation to or upon human tissues; to provide impression materials,which, while attaining each of the other stated objects of the inventionis substantially inert andpossessed of a controlled rate of gelation bythe mere addit on of water to the composition.

This and other objects are accomplished by utilizing a source orreservoir of ions from a first material which is normally non-reactivewith polycarboxy-polyhydroxy carboxymethyl cellulose or alginic acidmaterial, and a water soluble salt forming ions which are reactive withthe first material at a timed rate to liberate from said reservoir ofnon-reactive ions, ions which will precipitate and be reactive with thepolycarboxy polyhydroxy material to form a gel; by utilizing awater-soluble metal chelate which is substantially non-reactive with thep lyhy r y polycarboxy material, the precipitating ion from said chelatemay be liberated by displacing said precipitating ion with a salt of adisplacing ion which is more tightly bound to the chelate than theprecipitating ion and thereby causes the precipitating ion to react withpolycarboxypolyhydroxy material to form a gel at a controlled rate.

Still further objects and advantages of the present invention willappear from the more detailed description set forth below, it beingunderstood that such more detailed description is given by Way ofillustration and explanation only, and not by way of limitation, sincevarious changes therein may be made by those skilled in the art withoutdeparting from the scope and spirit of the present invention.

In carrying out the invention in an illustrative embodiment, there areprovided a composition and a method of forming and of using thecomposition to form gels of adjustable, superior mechanical andreproducing qualities, such as are essential in the critical field ofimpression materials to which the invention pertains.

The composition contains in homogeneous admixture the water-solublegelforming polyhydroxylic polyacid or its salts, such as the alginate orcarboxymethyl cellulose water soluble alkali metal salts and awater-soluble chelate of a coagulating ion for the gel-forming material,said coagulating ion releasable from said chelate at a beneficial rateunder minimum constant conditions of hydration to produce uniformstrength and elongation in the coagulum after release by the action of adisplacing ion more tightly bound to said chelate than said coagulatingion.

While the ability of many polycarboxylates, or their derivatives, togel, has been recognized heretofore in a broad sense, the specificadaptation of these materials to the critical requirements for dentalimpression materials has not met with the desired success.

The invention herein thus recognizes that these materials, i. e. thepolycarboxylates will not give all of the necessary properties asimpression compounds, nor furnish the high fidelity of reproductionessential for good dental impressions, unless attention is given to:

(a) The number and distribution of carboxy groups and hydroxyl groups onthe chain of the polymer. This ratio or relation basically controls gelstrength and syneresis i. e. contraction of the gel expressing water,and the rate of gelation.

(b) The molecular weight of the polymer.

(0) The type of the gelling ions and the amount used.

(d) The actual rate of gelation and its control to the desire of theultimate user.

(e) The use of particular salts to retain solvation of hydroxyls, or theuse of hygroscopic salts to balance syneresis.

(f) The use of salts and ions which do not interfere with setting ofplaster-of-Paris or artificial stones. (Certain ions and compositionsmay change plaster crystal habit, absorb components from the plaster, ormigrate into the plaster, forming irregular or soft plaster or stonesurfaces, or completely preventing setting thereof.)

(g) The mixability and wettability of the impressionpowder mix.

(h) The capability of long time storage stability before mixing. (Theso-called shelf life) In carrying out this invention, in a one-phase oranization, that is one that requires only the addition of water to formthe complete composition, with careful observance of the foregoingfactors, a highly effective impression material is produced.

The basic constituent of the impression material is a natural polymericmaterial such as alginate or modified natural polymeric material(carboxymethyl cellulose) which contains free hydroxyl groups and freecarboxyl groups. Illustratively, this may comprise a modified cellulosicmaterial produced by carboxylating cellulose, and which material isknown in the art and in its commercial forms as alkali; neutral oracid," carboxymethyl cellulose, referred to hereinafter as CMC." Thismaterial is marketed in various grades, based on molecular weight andthe number of substitutions of carboxyls per glucose unit; from 1 to 1.3carboxyl groups per recurring anhydro-glucose unit provides satisfactorydis- E solving mixing and setting characteristics. Thesepolyhydroxyl-polycarboxy materials with the just recited characteristicswill precipitate to form fair (but inadequate) gels under certainconditions with a number of divalent or higher valent ions. However,they coagulate to form a strong gel, with certain specific ionsdescribed hereinafter.

While the use of calcium, lead and aluminum salts have been previouslymentioned in the art of formulating algimate and carboxymethylcelluloseimpression material, many of these formulations require the use of anexcess of the precipitating ions to allow a sufficiently rapid reactionby mass action, and hence the completed gel tends to be too highly boundand shows strong syneresis. In contrast to this, the invention controlsthe rate and amount of precipitation or gelation in a unique doubledecomposition replacement system.

The controlled release of the divalent or higher valent ions is achievedby the use of a water soluble primary metal chelate. The metal in achelate is so complexed as not to be susceptible to precipitation bymany of the common precipitating agents or by thepolyhydroxy-polycarboxy acids used for the gel formation. The complexedmetal in the chelate, however, may be displaced from the chelate by ametallic ion from a salt which will not strongly gel CMC, but which iscapable of forming a stronger secondary chelate compound by virtue ofthe solubility product relationship. The original metal from the complexor reservoir is defined as the gelling ion, and the metal whichdisplaces the gelling ion in the original or primary chelate to form anew secondary chelate is defined as the displacing ion. synchronouslywith the formation of the new secondary chelate, the gelling ion isfreed to react with the CMC.

The rate of release of the gelling ion, and thus the rate of gelationitself, can be controlled by the type and concentration of thedisplacing ion. The displacement reaction is governed by the differencein dissociation constant of the primary and secondary chelates, and bytheir relative amounts. If the displacing ion forms a much more stronglybound chelate than the displaced gelling ion, the rate of displacementwill be high. If the displacing ion is at the same time present inamounts smaller than the displaced ion, then a part release will occurup to the amount of the displacing ion, and then the reaction will stop..As the concentration of the displacing ion is increased, eventually thetotal of the displaced ion will leave the chelate, and then the reactionstops. The same gelation at slower speeds can be obtained with thedisplacing ion forming a secondary chelate of higher dissociationconstant which is not as firmly bound as in ti e foregoing example, butis still morestrongly'bound than the primary chelate of the ion to bedisplaced. A further control is then obtainable by using larger amountsof this weaker displacing ion.

In brief, by selecting the appropriate dissociation constants andconcentrations, this system of ion supply from the reservoir of thechelates permits a more sensitive control over rate and extent ofrelease of the gelling ion than obtained heretofore, and presents anovel and important improvement in the art of gel formation for dentalirnpression materials.

Previous experimenters in the art have sought to effect the control byusing particular ions selected 'withspecial regard to the solubility ofthe salt used. The selected salt, however, has had other undesirablecharacteristics such as poor stability, high toxicity, or reactions withstone or plaster surfaces. The mechanisms for the release of theseselected ions often produces excessive density of crosslinks and hencehigh syneresis. Altogether the previous efforts to effect gelation havenot been found to be satisfactory.

Following are features of selection and control of the composition andprocess variables in accordance with the invention.

(1) Predetermining the nature of chelating compound.

(2-) Predetermining the degree of saturation of the primary chelate withregard to its metal.

(3) Predetermining the percentage of the primary chelate employed.

(4) Regulation of the pH of the solution by suitable buffers.

(5 Addition of a salt which will compete for either the gelling ion orthe displacing ion, the union of which would retard the reaction. Theaddition of a borate or an acetate, for example, retards the reaction,and as the pH of the mixture is gradually lowered, this activity isgradually lost, and the ion which has been affected is freed-toreact.

(6) Addition of one or more types of retarding agents, such as acitrate, tartrate, oxalate, or a phosphate. The etficiency of thepolyphosphates varies widely depending upon what materials are presentas precipitating agents. The polyphosphates may exert a threshold effectwhich allows these materials to temporarily inhibit the precipitation ofa metal ion even though there is not enough of the phosphate tocompletely complex the metals present.

(7) Employment of replacement ions in the form of salts, according totheir dissociation constants and the of the resultant mixture. Forexample, with ethylene diamine tetra acetic acid, a pH range of5.007.50, the decreasing order of the chelating strength is ferric ionion (Zn++.), and aluminum ion (Al+++).

(8) Variation of the types and amounts of fillers.

(9) Variation of the dilution by water, and the temperature thereof.

According to an illustrative example of the invention, the mixture is adry mass comprising carboxymethyl cellulose as polymeric materialcontaining hydroxyl groups and carboxyl groups which may be any of agroup comprised of either a natural polyhydroxypolycarboxy material suchas alginate or pectin, a modified natural material such as carboxymethylcellulose or any other carboxyalkyl cellulose, oxidized cellulose orphosphorylated cellulose or synthetic polyhydroxypolycarboxy polymerssuch as hydrolyzed copolymer of methacrylic acid, vinyl acetate so thatfree acid groups and free hydroxyl groups are formed, a soluble chelatesalt formed with a trivalent metal such as the cerium compound formed byreacting cerium sulfate with potassium ethylene diamine tetra acetate, afiller, a retarding agent, a soluble displacing metal salt other than analkali metal salt or alkaline earth salt, such, illustratively, asnickel sulfate. This mass reacts to form a gel upon the addition .ofwater, and establishes a controlled 'gelation rate and a gel of adequatedimensional strength and stability in accordance with theinvention.

A typical and illustrative composition, according to the above.procedure of a one-phase mixture, may comprise the following in theproportions shown by weight.

Percent l. CMC 1.20 carboxyl substitutions per glucose unit, highmolecular weightfl 2.09 2. Cerium potassium ethylene diamine tetraacetate 5.01 3. Potassium ethylene diamine tetra acetate 2.25 4.Diatomaceous earth 17.62 5. Nickel sulfate 2.54 6. Water 70.49

nickel ion (Ni++), cobalt ion (Co++), zinc be understood, isrepresentative of a large number that may be used.

Item 4 is an illustrative filler and can be replaced by others withappropriately changed proportions, as is well known in the art.

Item 5 is an illustrative displacing salt, of which several can be used,a few illustrative cations of salts being mentioned elsewhere in thisapplication.

The mixture of the above composition with water provides a mass withtimeand temperature-controlled gelation, adequate for proper mixing ofthe mass, application thereto to a support, insertion into the patientsmouth, forming the mixture about the teeth or gums or both, setting-upof the mixture into an elastic gel of substantial dimensional stability,and removing the formed gel as an integral imparted-form-retaining mass,from which an accurate plaster-of-Paris or artificial stone model can beobtained.

In the foregoing example, carboxy methyl cellulose,

molecular weight 243 at 1.2 carboxyl substitution, provides a molecularequivalent of 0.01. Cerium potassium ethylene diamine tetra acetate,molecular weight 467, has a divalent molecular equivalent of 0.02 and atrivalent molecular equivalent of 0.03 whereas the displacing ions ofnickel from nickel sulfate, molecular weight 263 furnishes a molecularequivalent of 0.02. Thereby there is displaced by the nickel ions anequivalent amount of cerium ions for gelling the CMC.

The illustrative composition contains about 70% water, and on a drybasis about 5 parts of the soluble cerium chelate mixed with about 2parts of restrainer and about 2 /2 parts of nickel sulfate displacingsalt provide the gelling ions for about 2 parts of CMC which is extendedwith about 18 parts of diatomaceous earth as a filler.

According to a second organization of a one-stage mixture, furtherillustrative of the invention, a dry mass is formed of the polymericmaterial, such as CMC or alginate, containing hydroxyl groups andcarboxyl groups, a soluble chelate formed with a higher valent metalsuch as the zirconium compound formed by reacting a zirconium salt withpotassium ethylene diamine tetra acetate, a filler, a retarding agent,and a soluble metallic salt other than an alkali, to wit, forillustrative purposes, zinc sulfate used in appropriate amounts toprovide a strong resilient gel. This mixture also establishes thedesired controlled gelation rate of a gel which is adequate and properfor dental impression according to the recited requirements thereof.

Percent l. CMC 4.009 2. Ferrous oxalate 0.501 3. Starch 4.009 4. Cottonflock 1.002 5. Sodium citrate 0.301 6. Water 80.178

Of the above, item 1, as noted, is a polymeric material which containsin it free hydroxyl groups and free carboxyl groups. Any other materialconforming with relatively high molecular weight within this definitioncan be used in substitution for the CMC.

Item 2 can be replaced with any other salt containing an ion which isreadily oxidized to a higher valency level at which it possesses gellingpower.

Items 3 and 4 represent illustrative fillers and can be replaced byothers such as are well known in the art.

Item 5 is an illustrative butfer, of many that may be used, as will beunderstood.

It will be recognized that the given percentages of phase or stage 1 donot total 100%, and the deficiency is supplied by the proportions of thetriggering solution of phase or stage 2, which follows, in anillustrative mixture:

Percent Chromic acid 0.500 Water 9.500

Mixture of phases one and two provides a mass of timeanddegree-controlled gelation, adequate for mixing of the mass, applicationthereof to a support, insertion in the patients mouth, forming themixture about the teeth and gums, setting-up of the mixture into anelastic gel of substantial dimensional stability, and removing theformed gel as an integral imparted form-retaining mass.

Although all of the organizations of impression material described andtheir reasonable equivalents in constituents, according to the Wellknown bases for chemical equivalence are available and highly operativefor the intended purposes of the invention, it will be understood thatfor general utility and convenience it is easier to obtain and mix waterwith a given composition of cooperative ingredients than it is topackage and open two related packages as in the last described two-stageform of the invention, and for general commercial purposes the onestagetype of invention is presently preferred.

Having thus described the invention, what is claimed is:

l. A method of producing a water-containing gel from an aqueous solutionof a gel-forming polyhydroxypolycarboxy material selected from the groupconsisting of carboxy-rnethyl cellulose, an alginate and a pectate,comprising uniformly incorporating in said solution a solublemulti-valent metal salt of ethylene diamine tetra-acetic acid, themulti-valent salt being chelated by said ethylene diamine tetra-aceticto prevent gelling of said gel-forming material, and incorporating awater-soluble divalent metal salt which releases a divalent metal ionhaving a higher chelating strength for said ethylene diamine tetraacetic acid than said multi-valen: metal salt of ethylene diaminetetra-acetic acid whereby said multi-valent metal salt ion is displacedby said divalent metal ion and precipitates the gel forming material.

2. A method as claimed in claim 1 wherein said multivalent salt is acerium potassium salt of ethylene diamine tetra-acetic acid and saiddisplacing divalent metal salt is nickel sulfate.

3. A method as claimed in claim 1 wherein said multivalent salt is analuminum salt of ethylene diamine tetraacetic acid and said divalentsalt is zinc sulfate.

4. A method as claimed in claim 1 wherein said multivalent salt is aferric salt of ethylene diamine tetraacetic acid and said divalent saltis nickel sulfate.

5. A process as claimed in claim 1 in which there is present an inertfiller.

6. A process as claimed in claim 1 in which there is present an alkalimetal salt of ethylene diamine tetraacetic acid as a restrainer.

7. An impression material consisting essentially on a dry weight basisof about 2 parts of a gel-forming polyhydroxy-polycarboxy materialselected from the group consisting of carboxy methyl cellulose, analginate and a pectate, about 5 parts of a water-soluble multi-valentmetal salt of ethylene diamine tetra-acetic acid, the multivalent saltbeing chelated by said ethylene diamine tetraacetic acid to preventgelling of said gel-forming material, about 2 parts of a water-solubledivalent metal salt which releases a divalent metal ion having a higherchelating strength for said ethylene diamine tetra-acetic acid than saidmulti-valent metal salt of ethylene diamine tetraacetic acid wherebysaid multivalent metal salt ion is displaced by said divalent metal ionfrom the ethylene diamine tetra-acetic acid chelate to precipitate thegel-forming material, and about 18 parts of an inert filler.

8. An impression material as claimed in claim 7 wherein saidmulti-valent salt is a cerium potassium salt of ethylene diaminetetra-acetic acid and said displacing divalent metal salt is a nickelsulfate.

9. An impression material as claimed in claim 7 wherein said multivalentsalt is a zirconium salt of ethylene diamine tetra-acetic acid and saiddivalent salt is zinc sulfate.

10. An impression material as claimed in claim 7 wherein saidmultivalent salt is a ferric salt of ethylene diamine tetra-acetic acid.

9 10 11. An expression material as claimed in claim 7 References Citedinthe file of this patent wherein said multivalent salt is an aluminumsalt of ethylene diamine tetra-acetic acid and in which there is TESTATES PATENTS present an alkali metal salt of ethylene diamine tetra2249'694 wlldufg July 15, 1941 acetic as a restrain 2,657,971 LochridgeNov. 3, 1953 12. An impression material as claimed in claim 7 whereinthere is present a polyphosphate as a retarding agent.

7. AN IMPRESSION MATERIAL CONSISTING ESSENTIALLY ON A DRY WEIGHT BASISOF ABOUT 2 PARTS OF A GEL-FORMING POLYHYDROX-POLYCARBOXY MATERIALSELECTED FROM THE GROUP CONSISTING OF CARBOXY METHYL CELLULOSE, ANALGINATE AND A PECTATE, ABOUT 5 PARTS OF A WATER-SOLUBLE MULTI-VALENTMETAL SALT OF ETHYLENE DIAMINE TETRA-ACETIC ACID, THE MULTIVALENT SALTBEING CHELATED BY SAID ETHYLENE DIAMINE TETRAACETIC ACID TO PREVENTGELLING OF SAID GEL-FORMING MATERIAL ABOUT 2 PARTS OF A WATER-SOLUBLEDIVALENT METAL SALT WHICH RELEASES A DIVALENT METAL ION HAVING A HIGHERCHELATING STRENGTH FOR SAID ETHYLENE DIAMINE TETRA-ACETIC ACID THAN SAIDMULTI-VALENT METAL SALT OF ETHYLENE DIAMINE TETRAACETIC ACID WHEREBYSAID MULTIVALENT METAL SALT ION IS DISPLACED BY SAID DIVALENT METAL IONFROM THE ETHYLENE DIAMINE TETRA-ACETIC ACID CHELATE TO PRECIPITATE THEGEL-FORMING MATERIAL, AND ABOUT 18 PARTS OF AN INERT FILLER.